The process of sampling is an important part of any analysis or measurement where the actual analysis or measurement is made outside of the volume actually containing the process or medium to be studied.
In FIG. 1. a prior art solution for sampling is presented. The object is to study characteristics of a process inside volume 11. A probe 12 is inserted to the volume 11 through which a sample is transferred to a pre-processor means 13 outside the volume 11. In the pre-processor means 13 the extracted sample is processed so that it is better suited for the actual measurement carried out by the analyser 14. Some possible pre-processing processes comprise lowering the temperature of the sample and diluting.
Pre-processing is an important step of the sampling as it is the object to measure the process in volume 11 and as the pre-processing happens before the actual analysis all the modifications made in the pre-processing effects the result of the analysis. Thus a good pre-processing step is such that it will have a minimal effect on the characteristics to be analysed, but at the same time carrying out the modifications of the non-essential characteristics so that the sample can be analysed by the analyser.
Diluting is one of the most important methods of sample pre-processing for gaseous medium. In a dilution process air, or some another dilution medium, is mixed with a sample flow. The dilution ratio, i.e. ratio between dilution flow and sample flow, varies quite a lot depending e.g. of the analysis method to be used. For example in flue gas analysis dilution ratios 10-100 are commonly used. Dilution ratio is one of the major parameters to keep track of because it has direct effect on the measurement result as the measurement is made on the diluted sample.
Another important parameter of pre-processing is temperature. This is especially important when analyzing gaseous samples prone to chemical reactions or vapour condensation. These kind of measurements comprise e.g. exhaust gas measurement.
Thus a good sample pre-processing process for a hot gaseous medium would be such that it would allow good control over dilution ratio and temperature.
The prior art solutions include use of diluters based on porous tubes. One example is presented in FIG 2. The sample flow 21 is directed into a central cavity 29 of a porous tube. At the same time dilution air 22 is directed to the tube so that it will enter the central cavity 29 through the walls of the cavity. Sample flow 21 and dilution flow 22 will mix and produce a diluted flow 23 which is directed to an analyser 14.
One problem of the prior art porous tube diluters is the difficulty to adjust the dilution ratio with a high precision. Also controlling the temperature of the diluted flow is difficult with a prior art porous tube dilutor.
In FIG. 3 another prior art solution for diluting a sample is presented. The diluter type presented in FIG. 3 is called an ejector. In an ejector type configuration the dilution flow 22 is ejected into sample flow 21 typically in a quite a limited point of space. Sample flow 21 and dilution flow 22 mix and produce a diluted flow 23, which is then directed to an analyser 14. Problems of the prior art type ejectors relate to fouling and losses.